Many communication systems modulate electromagnetic signals from baseband to higher frequencies for transmission, and subsequently demodulate those high frequencies back to their original frequency band when they reach the receiver. The original (or baseband) signal may be, for example: data, voice or video. These baseband signals may be produced by transducers such as microphones or video cameras, be computer generated, or be transferred from an electronic storage device. In general, the high frequencies provide longer range and higher capacity channels than baseband signals, and because high frequency signals can effectively propagate through the air, they can be used for wireless transmissions as well as hard wired or wave guided channels.
All of these signals are generally referred to as RF signals, which are electromagnetic signals; that is, waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation.
Wired communication systems which employ such modulation and demodulation techniques include computer communication systems such as local area networks (LANs), point-to-point communications, and wide area networks (WANs) such as the Internet. These networks generally communicate data signals over electrically conductive or optical fibre channels. Wireless communication systems which may employ modulation and demodulation include those for public broadcasting such as AM and FM radio, and UHF and VHF television. Private communication systems may include cellular telephone networks, personal paging devices, HF radio systems used by taxi services, microwave backbone networks, interconnected appliances under the Bluetooth standard, and satellite communications. Other wired and wireless systems which use RF modulation and demodulation would be known to those skilled in the art.
Most RF receivers use the “super-heterodyne” topology, which provides good performance in a limited scope of applications, such as in public-broadcast FM radio receivers. The super-heterodyne receiver uses a two-step frequency translation method to convert a received RF signal to a baseband signal which can be played using an audio amplifier and speaker, for example. FIG. 1 presents a block diagram of a typical super-heterodyne receiver 10. The mixers 12 and 14 are used to translate the RF signal to baseband or to some intermediate frequency (IF). The balance of the components amplify the signal being processed and filter noise from it.
The RF band pass filter 18 first filters the signal coming from the antenna 20 (note that this band pass filter 18 may also be a duplexer). A low noise amplifier 22 then amplifies the filtered antenna signal, increasing the strength of the RF signal and reducing the noise figure of the receiver 10. The signal is next filtered by another band pass filter 24 usually identified as an image rejection filter. The signal then enters mixer 12 which multiplies the signal from the image rejection filter 24 with a periodic signal generated by a local oscillator (LO1)26. The mixer 12 receives the signal from the image rejection filter 24 and translates it to a lower frequency, known as the first intermediate frequency.
Generally, a mixer is a circuit or device that accepts as its input two different frequencies and presents at its output:    (a) a signal equal in frequency to the sum of the frequencies of the input signals;    (b) a signal equal in frequency to the difference between the frequencies of the input signals; and    (c) signals at the original input frequencies.The typical embodiment of a mixer is a digital switch which may generate significantly more tones than those stated above.
The first intermediate frequency signal is next filtered by a band pass filter 28 typically called the channel filter, which is centred around the first intermediate frequency, thus filtering out the unwanted products of the first mixing processes; signals (a) and (c) above. This is necessary to prevent these signals from interfering with the desired signal when the second mixing process is performed.
The signal is then amplified by an intermediate frequency amplifier 30, and is mixed with a second local oscillator signal using mixer 14 and local oscillator (LO2) 32. The second local oscillator LO2 32 generates a periodic signal which is typically tuned to the first intermediate frequency. Thus, the signal coming from the output of mixer 14 is now at baseband, that is, the frequency at which the signal was originally generated. Noise is now filtered from the signal using the low pass filter 34, and the filtered baseband signal is passed on to some manner of presentation, processing or recording device. For example, in the case of a radio receiver, this might be an audio amplifier and speaker, while in the case of a computer modem this may be an analogue to digital convertor.
Note that the same process can be used to modulate or demodulate any electrical signal from one frequency to another.
The super-heterodyne design 10 suffers from a number of problems including the following:                it requires expensive off-chip components, particularly band pass filters 18, 24, 28, and low pass filter 34;        the off-chip components require design trade-offs that increase power consumption and reduce system gain;        it is not fully integratable; and        it is not easily applied to multi-standard/multi-frequency applications because the band pass and low pass filters 18, 24, 28 and 34 must be high quality devices; electronically tunable filters cannot be used. The only way to use the super-heterodyne system in a multi-standard/multi-frequency application is to use a separate set of off-chip filters for each frequency band.        
Direct conversion architectures are different from super-heterodyne architectures in that they demodulate RF signals to baseband in a single step. By mixing the RF signal with a local oscillator signal at the carrier frequency, there is no image frequency, and no image components to corrupt the signal. Direct conversion receivers offer a high level of integratability, but also have several important problems.
A typical direct conversion receiver 36 is presented in the block diagram of FIG. 2. The RF band pass filter 18 first filters the signal coming from the antenna 20 (this band pass filter 18 may also be a duplexer). A low noise amplifier 22 then amplifies the filtered antenna signal, increasing the strength of the RF signal and reducing the noise figure of the receiver 36.
The signal is then mixed with a local oscillator signal using mixer 14 and a local oscillator 38. The local oscillator 38 generates a periodic signal which is tuned to the carrier frequency of the received signal, rather than an IF frequency as in the case of the super-heterodyne receiver 10. The signal coming from the output of mixer 14 is now at baseband, that is, the frequency at which the received signal was originally generated. The down-converted signal is then filtered using low pass filter 34, and may be amplified by amplifier 39. The baseband signal can now be amplified, digitized or converted in some other way, into a useful signal.
Direct conversion RF receivers 36 have several advantages over super-heterodyne systems in term of cost, power, and level of integration, however, there are also several serious problems with direct conversion. These problems include:                noise near baseband (that is, 1/f noise) which corrupts the desired signal;        local oscillator (LO) leakage in the RF path which creates DC offsets. As the LO frequency is the same as the carrier frequency of the incoming signal being demodulated, any leakage of the LO signal onto the antenna side of the mixer will pass through to the output side as well;        local oscillator leakage into the RF path which causes desensitization. Desensitization is the reduction of desired signal gain as a result of receiver reaction to an undesired signal. The gain reduction is generally due to overload of some portion of the receiver, such as automatic gain control (AGC) circuitry, resulting in suppression of the desired signal because the receiver will no longer respond linearly to incremental changes in input voltage;        RF-LO leakage can also couple to the on-chip voltage controlled oscillator (VCO) used to generate the local oscillator signal and degrade receiver performance, especially in phase-modulated systems; and        large on-chip capacitors are required to remove unwanted noise and signal energy near DC, which makes integrability expensive. These capacitors are typically placed between the mixer 12 and the low pass filter 34.Clearly, the problems listed above can be reduced significantly if the signals can be effectively isolated from one another. Unfortunately, this is very difficult to do in a cost effective and efficient way.        
In “Solving the Direct Conversion Problem,” Planet Analog, August 2001, D. Grant et al. propose a number of ways to reduce LO leakage including the following:                good board design (for example, minimizing the length of LO traces to keep traces from acting like “antennas” which broadcast the LO signal);        generous shielding, which can add cost and weight;        generating an LO signal at a multiple or factor of what is required, then using a divider or multiplier to produce the actual LO where it is needed.Grant et al. also describe the use of a simple regenerative divider circuit to produce an LO at 4/3 of the desired LO frequency, in an effort to avoid LO leakage problems.        
However, Grant et al. do not offer any effective way of implementing such a circuit in an I and Q application. In many modulation schemes (particularly frequency modulation and phase modulation schemes), it is necessary to modulate or demodulate both in-phase (I) and quadrature (Q) components of the input signal, where I is 90 degrees out of phase with Q.
Simply modifying the Grant design to handle I and Q signals in the manner known in the art results in a circuit with almost as many filters and other components as required in previously known topologies. Thus, Grant et al. offer no real improvement in terms of integrability, cost and size reduction in applications requiring I and Q signals.
There is a great desire to provide modulation and demodulation circuits in a completely integrated form in the interest of providing smaller, lighter devices which are less expensive, and which consume less power. Discrete electronic components such as off-chip filters, are physically large, comparatively expensive and consume more power than integrated components. Clearly, topologies which require such off-chip components are undesirable.
However, fabricating fully-integrated receivers using cost-effective fabrication technologies is not without challenges. CMOS technology, for example, offers passive components with low quality factor and low self-resonant frequencies, which can pose problems, particularly at higher frequencies.
The continuing desire to implement low-cost, power efficient transmitters has proven especially challenging as the frequencies of interest in the wireless telecommunications industry (especially low-power cellular/micro-cellular voice/data personal communications systems) have risen above those used previously (approximately 900 MHz) into the spectrum above 1 GHz.
Thus, there is a need for a method and apparatus for demodulation which addresses the problems above. It is desirable that this design be fully-integratable, inexpensive and high performance.